#include <stdio.h>

int atoi(const char *s)
{
    int result = 0;
    
    int sign = 1;
    
    if( *s == '-')
    
    {
        sign = -1;
    }
    
    while(*s != '\0')
    {
        if(*s >= '0' && *s <= '9')
        {result= result * 10 + *s-'0';}
        
        s++;
    }
    
    return result * sign;
}


int main()
{

    const char *s = "1234";
    int a = atoi(s);
    printf("%d\n",a);
    return 0;
}